Oxfordshire Assistant Chief Fire Officer Simon Furlong said at the scene: “The remainder of the building is very unsafe, which is hampering the search. This is a very difficult situation with a very unstable structure.

“The safety of emergency service personnel has to remain our priority, while recognising how hard this must be for families waiting for news of loved ones overnight. Our sympathies are with them, and the family of the person who died here yesterday.

“An expert from Cheshire with similar experience is due on site today to advise on the search.

“The police will be taking over control of the site as part of ongoing investigation but we are continuing working with fire services from Buckinghamshire, Hampshire and the West Midlands, and this work may continue for several days.”

A cordon has been placed around the scene as the rescue operation continues.

Deputy Chief Fire Officer Nathan Travis described the site as “challenging”.

He said: “The building is potentially 10 storeys high; half of that building has collapsed, so you have got a rubble pile which is approximately 20 to 30 feet deep at the moment.

“The search will be considerable due to the instability of the site; we expect the search to continue throughout the night and possibly into the coming days.”

Asked about the chances of finding the missing alive, he replied: “At the moment I can’t give you any details on that but it is a substantial collapse of a building.”

Pictures from the scene showed a significant chunk of a building in the defunct Didcot A site has collapsed, with a large amount of debris on the ground.

A GMB union official told the Press Association that workers were preparing two boilers for demolition in the coming week, and that is believed to have led to the collapse of the building.

People in Oxfordshire have been asked to consider whether they require emergency hospital care after a major incident was declared.

Oxford University Hospitals NHS Foundation Trust statement said: “We are grateful to members of the public for avoiding attending our Emergency Department for anything other than serious or life-threatening emergencies.”

Witness David Cooke, whose company, Thames Cryogenics, has a building overlooking the power station, said: “Our building shook and as we looked out of the window. The end of the main turbine hall collapsed in a huge pile of dust.

“It totally obscured the towers and must have drifted across the roads and main rail line. What’s left looks a tangled mess.

“The dust was hanging over the area for five to 10 minutes.”

Didcot A opened in 1970 as a coal-fired power station and was later converted so it could also generate power from natural gas.

It ceased generation in March 2013 and hundreds gathered to watch when three of its enormous cooling towers were blown up in July 2014 after dominating the town’s skyline for more than four decades.

The incident comes 16 months after a major fire struck a cooling tower at Didcot B in October 2014.

The blaze affected 50 per cent of the station output, supplying a million homes.